I agree that the SIM must be provisioned. I’ve gotten several new sims over the years when I upgraded my iPhones.My 6S is using a Straight Talk SIM and it still doesn't have VoLTE. It's 'enabled' in settings, but call quality is low enough to recognize it as NOT being VoLTE. Same with my Galaxy S5 before that. I've noticed that '1x' also displays as the data type at work, as well as during calls.
Doesn't work if your SIM was never provisioned for VoLTE.
AT&T spells out the requirements below

Get Higher Voice Quality With AT&T HD Voice
Enjoy crystal-clear conversations and reduced background noise with AT&T HD Voice on our network.
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